Field of the Invention
The invention relates to a method for operating a hybrid motor vehicle which has an internal combustion engine and at least one electric machine.
Conventional hybrid vehicles, which have an electric machine (for example a starter generator) between an internal combustion engine and a transmission, are known. In the case of manually shifted transmissions, the electric machine can usually be disengaged from the transmission by a clutch, wherein this clutch also serves as a starting clutch.
At least when the vehicle is at a standstill, but also during phases of deceleration, the internal combustion engine can be turned off in order to save fuel in these vehicles. German Patent Application Publication No. DE 100 40 094 A1 discloses an automatic start-stop-controlling/regulating device for an internal combustion engine of a motor vehicle. The restart of the internal combustion engine is performed by the electric machine, also called the starter generator, at the latest when a torque is requested by the driver. Since hybrid vehicles have substantially more powerful electric motors as compared to conventional starter motors, the restart of the engine is possible with a high degree of comfort. Due to the high electromotive torque, it is possible to drag the internal combustion engine within a short period of time up to its starting speed or idle speed, such that the start delay is barely noticeable for the driver. In addition, the starting process is also acoustically more comfortable, because the typical meshing noise and running noise of a conventional starter is no longer present.
Nevertheless the start or restart of the internal combustion engine after a stop-phase is also noticeable in hybrid vehicles, which is attributed mainly to the oscillating torques that are created when cranking the internal combustion engine. The oscillating torques are caused by the individual operating cycles of the internal combustion engine, mainly the compression phase and the subsequent expansion phase. They depend in particular on the number of strokes (two stroke engine or four stroke engine), on the number of cylinders, on the control timing and the compression ratio. Furthermore, oscillating torques are caused by the oscillating inertial forces.
In conventional controls of hybrid drive systems the running-up of the internal combustion engine during start is performed by a speed control method or torque control method, in order to follow a given setpoint course or target course with smallest possible deviations. The global setpoint speed course or setpoint torque course predefines in this case how the internal combustion engine runs from standstill up to its starting speed or idle speed. The influence of oscillating torques is not included in these global setpoint courses. To be precise, deviations from the setpoint course are in principle compensated by a superimposed closed-loop control, in practice however, the oscillating torques generate disturbance variables for the closed-loop control such that in particular in case of the speed control of the electric machine but also in case of a torque control, control deviations occur (even in case of a constant feedforward control for the torque, fluctuations in the torque occur due to changing reaction torques of the internal combustion engine). For this reason, there is a potential for improvement in comfort for the highly dynamic, short start procedure of the internal combustion engine.
In case of a drive system with an electric machine it is known from German Patent No. DE 198 14 402 C2 to give the electric machine a double function in the first phase of the driving-away process. On the one hand, the electric machine serves to accelerate the vehicle when driving away, on the other hand, the electric machine cranks up the internal combustion engine for starting. In addition, the electric machine generates an oscillating torque which has the same absolute value and is opposite in phase to the torque fluctuations which the internal combustion engine generates when being dragged along.
German Patent Application Publication No. DE 102 01 278 A1 discloses a method for turning off an internal combustion engine with a defined position of the crankshaft.